Cambrian Explosion: role of oxygen (Introduction)

by David Turell @, Tuesday, February 23, 2016, 01:17 (1496 days ago) @ David Turell

In this essay Sci. Am. covers the rise in oxygen story using the same material presented about a week ago fro Nature, but in more detail:

"Some scientists now think that a small, perhaps temporary, increase in oxygen suddenly crossed an ecological threshold, enabling the emergence of predators. The rise of carnivory would have set off an evolutionary arms race that led to the burst of complex body types and behaviours that fill the oceans today. “This is the most significant event in Earth evolution,” says Guy Narbonne, a palaeobiologist at Queen's University in Kingston, Canada. “The advent of pervasive carnivory, made possible by oxygenation, is likely to have been a major trigger.”


"The latest results come at a time when scientists are already reconsidering what was happening to ocean oxygen levels during this crucial period. Donald Canfield, a geobiologist at the University of Southern Denmark in Odense, doubts that oxygen was a limiting factor for early animals. In a study published last month, he and his colleagues suggest that oxygen levels were already high enough to support simple animals, such as sponges, hundreds of millions of years before they actually appeared. Cambrian animals would have needed more oxygen than early sponges, concedes Canfield. “But you don't need an increase in oxygen across the Ediacaran/Cambrian boundary,” he says; oxygen could already have been abundant enough “for a long, long time before”.


"Cloudina were among the earliest animals known to have grown hard, mineralized exoskeletons. But they were not alone. Two other types of animal in those reefs also had mineralized parts, which suggests that multiple, unrelated groups evolved skeletal shells around the same time. “Skeletons are quite costly to produce,” says Wood. “It's very difficult to come up with a reason other than defence for why an animal should bother to create a skeleton for itself.” Wood thinks that the skeletons provided protection against newly evolved predators. Some Cloudina fossils from that period even have holes in their sides, which scientists interpret as the marks of attackers that bore into the creatures' shells.


"The rise of predation at this time put large, sedentary Ediacaran animals at a big disadvantage. “Sitting around doing nothing becomes a liability,” says Narbonne.


"This past autumn, Woods visited Siberia with that goal in mind. She collected fossils of Cloudina and another skeletonized animal, Suvorovella, from the waning days of the Ediacaran. Those sites gave her the chance to gather fossils from many different depths in the ancient ocean, from the more oxygen-rich surface waters to deeper zones. Wood plans to look for patterns in where animals were growing tougher skeletons, whether they were under attack by predators and whether any of this had a clear link with oxygen levels, she says. “Only then can you pick out the story.'”

Comment: Same comment. Environmental factors stimulate adaptations, but completely new animals are another issue, not required by the changes we now see. These are enormous changes with complete animals with complete organ systems and brains!

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